1. Principles of Technology
Change of Phase
Principles of Technology

2. Matter normally exists in one of three phases: solid, liquid, or gas.
Phase depends on the materials internal energy (as indicated by its temperature) and the pressure acting on the material.
Adding or removing heat is one means to change the phase of a material.
In the solid phase, molecules are held together by attractive forces (bonds).
Adding heat causes increased motion about the molecular equilibrium positions.

3. If enough heat is added to provide enough energy to break the molecular bonds, most solids undergo a phase change and become liquids.
The temperature at which the phase change occurs is the melting point.
The temperature at which a liquid becomes a solid is the freezing point.
These temperatures are the same for a given material, but they can differ slightly.
In the liquid phase, molecules of a material are relatively free to move and a liquid takes the shape of its container.

4. Certain liquids have locally ordered structure, creating liquid crystals such as those used in liquid crystal displays.
Adding heat to a liquid increases the motion of the molecules of the liquid.
When the molecules have enough energy to separate, the liquid changes to the gaseous or vapor phase.
The change may occur slowly be evaporation or rapidly at a particular temperature called the boiling point.
The temperature at which a gas condenses into a liquid is called the condensation point.

5. Some solids, such as dry ice, mothballs, and certain air fresheners, change directly from the solid to the gaseous phase at standard pressure.
The change from a solid directly to a vapor is called sublimation.
Like evaporation, the rate of sublimation increases with temperature.
Phase change from gas to a solid is called deposition.
Ex. Frost is solidified water vapor deposited on objects.
Frost is NOT frozen dew (liquid water).
Latent Heat
Generally, when heat is transferred to a material, its temperature increases as the average kinetic energy per molecule increases.

6. When heat is added or removed during a phase change, the temperature of the material does NOT change.
When adding heat to ice during a phase change, the addition of more heat does not increase the temperature of the ice, but causes the ice to melt (change phase).
The heat must be added slowly so that the ice and the melted water remain in thermal equilibrium, otherwise, the ice water can warm above  °C even tho the ice remains at  °C.
Only after the ice is completely melted does adding more heat cause the temperature of the water to rise.
A similar situation occurs during the liquid-gas phase change at the boiling point.
Adding more heat to boiling water only causes more vaporization.

7. A temperature change only occurs after the water is completely boiled, resulting in superheated steam.
During a phase change, the heat goes into breaking the attractive bonds and separating the molecules rather than increasing the temperature (increasing their potential rather than kinetic energies).
The heat required for a phase change is called the latent heat (L), defined as the magnitude of the heat needed per unit mass to induce a phase change:
Unit: J/kg or kcal/kg.
Latent heat for a solid-liquid phase change is the latent heat of fusion (Lf).
Latent heat for a liquid-gas phase change is the latent heat of vaporization (Lv).

8. Latent heat for the solid-gas phase change is the latent heat of sublimation (Ls).
The latent heat is the amount of energy per kg given p when the phase change in in the opposite direction, from liquid to solid or gas to liquid.

9. Equation: Q = ±m·L; the positive or negative sign must be expressed because heat can flow into (+) or out of (-) the material or system.
For water: Lf = 3.33 x 105 J/kg; Lv = 22.6 x 105 J/kg

10. Technically, the freezing point and boiling points of water ( °C and 100 °C) apply only at 1 atm pressure.
Phase-change temperatures generally vary with pressure.
The boiling point of water decreases with decreasing pressure; this is why water boils at a lower temperature at high altitudes where the air pressure is lower.
Lower pressure lengthens the cooking time of food.
Pressure cookers are used to reduce cooking time by increasing the pressure which raises the boiling point.
Freezing point of water decreases with increasing pressure.
Evaporation of water from open container only becomes evident after a relatively long period of time.
The molecules of the liquid are in motion at different speeds.
Faster moving molecules near the surface may momentarily leave the liquid.

11. If the speed is not too large, the molecule will return to the liquid because of the attractive forces exerted by the other molecules.
Molecules with a large speed can leave the liquid entirely.
The higher the temperature of the liquid, the more likely molecules will have the necessary speed to leave the liquid.
Escaping molecules take their energy with them, reducing the average molecular energy of the liquid and resulting in a lower temperature.
Evaporation is a cooling process for the object from which the molecules escape.

12. The rate of evaporation of a liquid depends on the:
Amount of surface area. The larger the surface area, the more molecules that have a chance to escape from the surface.
Temperature. The higher the temperature, the higher the molecular energy of the molecules, which allows more molecules to escape.
Surface currents. Air currents blowing over the liquid’s surface remove many of the molecules that have evaporated before they fall back into the liquid, which is why a cool summer breeze “feels so good”.

13. 4. Volatility. The volatility of a liquid is a measure of its ability to vaporize. Examples of a highly volatile liquid is rubbing alcohol and gasoline. The more volatile the liquid, the greater its rate of evaporation.
Pressure on or above the liquid. The lower the pressure, the greater the rate of evaporation. Under a partial vacuum, there are fewer molecules around which the liquid molecules may collide, allowing for a higher rate of escape and a higher rate of evaporation.
Humidity. If the liquid is exposed to the atmosphere, lower humidity values will provide for greater evaporation.

14. Heats of Transformation
When energy is absorbed as heat by a solid or liquid, the temperature of the object does not necessarily rise.
The thermal energy may cause the mass to change from one phase, or state, to another.
The amount of energy per unit mass that must be transferred as heat when a mass undergoes a phase change is called the heat of transformation, L.

15. Heat of Fusion
To melt a solid means to change its from the solid state to the liquid state. This process requires energy because the molecules of the solid must be freed from their rigid structure.
To freeze a liquid to form a solid is the reverse of melting and requires that energy be removed from the liquid so that the molecules can settle into a rigid structure.

16. Heat of Fusion
When the phase change is from solid to liquid, the sample must absorb heat; when the phase change is from a liquid to solid, the sample must release heat.
The heat of transformation for these phase changes is called the heat of fusion, Lf.
Water: Lf = 334 J/g = 79.5 cal/g

17. Heat of Vaporization Water: Lv = 2256 J/g = 539 cal/g
To vaporize a liquid means to change it from the liquid state to the vapor or gas state. This process requires energy because the molecules must be freed from the liquid state.
Condensing a gas to a liquid is the reverse of vaporizing; it requires that energy be removed from the gas so that the molecules can cluster together instead of flying away from each other.
The heat of transformation for these phase changes is called the heat of vaporization, Lv.
Water: Lv = 2256 J/g = 539 cal/g

18. Phase Changes

19. Gas-Liquid Equilibration

21. Phase Changes Energy Changes Accompanying Phase Changes
Copyright 1999, PRENTICE HALL
Chapter 11

23. Phase Changes
Heating Curves
Copyright 1999, PRENTICE HALL
Chapter 11

24. Heating Curve Illustrated
Copyright 1999, PRENTICE HALL
Chapter 11

27. Phase Change Diagram - Water

28. Latent Heat Latent heat of fusion / melting: 330,000 J/kg
E. gas
C. liquid
A. solid
Demo: ice water heated over bunsen burner (with thermometer)
Latent heat of fusion / melting: 330,000 J/kg
Latent heat of vaporization / evaporation: 2,260,000 J/kg

29. Heat of Combustion (HC)
The amount of heat released per unit mass or unit volume of a substance when the substance is completely burned.
Heats of combustion are used as a basis for comparing the heating value of fuels, since the fuel that produces the greater amount of heat for a given cost is the more economic.
Equation:

30. Effects of Pressure and Impurities on Change of Phase
Automobile cooling systems present important problems concerning change of phase.
Most substances contract on solidifying; however, water expands.
The tremendous force exerted by this expansion is shown by the number of cracked automobile blocks and burst radiators suffered by careless auto owners every winter.
Hence the need for antifreeze.
Impurities in water tend to lower the freezing point.

31. An increase in the pressure on a liquid raises the boiling point.
Alcohol has a lower freezing point than water and is used in some types of antifreeze.
By mixing antifreeze with water in the cooling system, the freezing point of the water may be lowered to avoid freezing in winter.
Automobile engines may also be ruined in winter by overheating if the water in the radiator is frozen, preventing the engine from being cooled by the circulation in the system.
An increase in the pressure on a liquid raises the boiling point.
Automobile manufacturers use this fact by pressurizing their cooling systems and thereby raising the boiling point of the coolant used.
A decrease in the pressure on a liquid lowers the boiling point.
Frozen concentrated orange juice is produced by subjecting the pure juice to very low pressure at which the water in the juice is evaporated.
The consumer must restore the lost water before serving the juice.

32. Phase Change Example
Heat is added to 0.5 kg of water at 20° C. How many joules of heat energy are required to change the water to steam at 110° C?
Heat has to be added to the water to raise its temperature from 20° C to the boiling point 100° C.
Heat has to be added to vaporize all of the water that is at 100° C.

33. Phase Change Example
Heat has to be added to raise the temperature of all of the steam from 100° C to 110° C.
Total heat energy: Q = J J J = J

34. Final Temperature Example
A 0.3 kg piece of ice at 0° C is placed in 1 kg of water at 40° C in an insulated container. If no heat is lost to the container, what is the final temperature of the water?
Q lost water = Q gained ice + Q gained ice water
(m·c·DT)water = m·Lf + (m·c·DT)ice water

35. Final Temperature Example